In addition to actually propelling an associated vehicle, the drive engines of conventional vehicles also serve to drive additional units and to generate electric power by means of a generator connected to the drive engine. Electric power is required to supply a variety of consumers, some of which have relatively high power uptake requirements. Exemplary consumers of this type include heaters, (e.g., window, passenger compartment, seat, and catalyst heaters), air conditioning compressors, and high-powered drives (e.g., electrically operated steering systems and electrically operated brakes). When the drive engine is not being used to propel the vehicle, it runs in an idling state. In the idling state, the drive engine need only perform the work required to drive its own rotation, the work required to drive any mechanical auxiliary units, and the work required to drive the generator. To keep fuel consumption, pollutant emissions and noise development low, the idling speed is generally chosen as low as possible and, in fact, is set just above a minimum speed. In order to ensure the engine does not run haltingly or even stop, this minimum speed should not be fallen short of, even for a brief time. In the idling state, the internal combustion engine has only a very limited power reserve.
In conventional systems, switching on an electrical consumer of high power leads to a sudden voltage drop in the electrical system (or another system via which the consumer is supplied). This voltage drop leads to a sudden rise in exciter current in the generator control to constant voltage, which is accompanied by a likewise sudden increase in the braking torque exerted by the generator on the internal combustion engine. Such an increase in braking torque causes a drop in the idling speed of the internal combustion engine.
Conventional idling speed control systems seek to counteract such drops by performing a so-called filling intervention and/or an ignition intervention (see, for example, Automotive Handbook/Bosch, 21st edition, 1991, p. 466). In a typical filling intervention, a drop in the idling speed is counteracted by injecting an increased amount of fuel into the engine. The internal combustion engine responds to the fuel increase by producing an increased drive torque. Unfortunately, a relatively long period of time typically elapses before the increased torque produced by the filling intervention is made available. Control is, therefore, sluggish in the filling intervention context.
In a conventional ignition intervention, a drop in the idling speed of the internal combustion engine is counteracted by adjusting the ignition point from "late" to "early" in the ignition cycle. Shifting the ignition point in this manner causes an increase in the drive torque produced by the engine. Ignition intervention is much faster than filling intervention (i.e., it has a much shorter response time). However, ignition intervention is disadvantageous in that the internal combustion engines employing ignition intervention must be generally operated with a late ignition point in the idling state in order to ensure there is sufficient latitude for ignition point adjustment to occur in the "early" direction during a drop in the idling speed.
Although both intervention solutions mentioned above function in principle, they are not considered optimal. In particular, because of the relatively sluggish response of filling intervention, to avoid a temporary drop below the minimum idling speed, the idling speed of internal combustion engines employing that technique must generally be placed a relatively large distance above the minimum speed. On the other hand, in engines employing ignition intervention, the internal combustion engine runs with reduced efficiency because, as mentioned above, the ignition point must be set late in the ignition cycle. Both of the conventional intervention solutions, therefore, cause increased fuel consumption and pollutant emissions in comparison with constant idle conditions.